Some of the ocean's most efficient swimming animals use suction to propel themselves through the water, finds a study in Nature Communications. Rather than generating thrust by propelling water backwards, jellyfish and lampreys are observed primarily pulling themselves through the water by generating low pressure regions in the fluid surrounding their bodies.

A central principle of all animal locomotion - on land, in the air, or in water - is that forward propulsion is generated by pushing back on the surrounding contact surface. In swimming animals, this propulsive thrust is generated by movements that create high pressure regions in the fluid against which the body can push.

John Dabiri and colleagues study the movements of two freely-swimming lampreys (and two whose swimming efficiency was reduced), and a single jellyfish as they move through tunnels where the water has been 'seeded' with small (ten microns in size) glass beads to allow the motion of the surrounding fluid to be accurately traced. Calculating the relative contributions of different regions of fluid pressure to forward and backwards motions, they find that low pressure regions dominate the balance of forces in the fluid immediately surrounding the animals' body.

These low pressure regions are generated by coordinated undulations of the body which create vortices that propagate down the length of the animals' body, generating forward pull and effectively sucking them through the water. Jellyfish and lampreys have some of the lowest energetic swimming costs of any animal, and it is hoped that their shared solution for efficient swimming could be used to help design future underwater vehicles.